A Posse Ad Esse
by Melboriel
Summary: James comes up with a ridiculous idea, all in jest, but Sirius takes it on wholeheartedly. This is one reason why Severus hates Sirius with such passion, and it all has to do with his twin brother.... slashiness SB/OC SB/?. rated for mature themes.
1. Chapter 1

**Disclaimer: Harry Potter and all its associated characters are © J. K. Rowling and Warner Bros. I'm not making any money from this little story. A shame really as I could do with some.**

**Summary:** Maurader Era fic. Ever wondered why the enmity between Sirius Black and Severus Snape is so high? No mere school bullying could account for its intensity. Well, imagine that Severus has a twin brother, and imagine that Sirius gets it into his head to seduce him… well this is the story of what happens next. Slashiness with Sirius/OC, hints of Sirius/?? And set in their 6th year at Hogwarts. Enjoy.

**Author's note:** So this is a first for me in many ways. 1) My first venture into the Harry Potter universe. 2) My first slash story; frankly my first story with any 'romance'. And 3) My first time writing from first person perspective. However, this story seemed to lend itself to that. So it's definitely starting off from Sirius's POV but I'll probably switch perspectives at some point. This story is all planned but certainly not all written out… anyway that's about it. I'm currently half way through the write up of my Master's Thesis and this was a nice little break from that but work is calling again… alas. The title is in Latin for no other reason than I couldn't really think of a snappy title and I like Latin. It means from possibility to actuality.

**A Posse Ad Esse**

**Chapter One: How it all Began.**

**_Sirius_**

It all started out as a bit of a joke, really; a passing remark made by James one evening whilst the four of us were discussing new ways to wind up Snivellus in this, our sixth year at Hogwarts.

"Y'know what would really wind him up, Padfoot?" said James lazily, lolling on one elbow on my bed and giving me a wink. "If you went about seducing his brother and let the whole school know.'

I almost choked on the pumpkin juice I was swigging. James had often come up with some wild schemes in the past but this topped the lot. Of course my reputation with the _laydees_ was a source of much amusement for my best friend, and he often claimed if I couldn't get the girl it couldn't be done. Obviously he had now broadened his horizons on that front.

Remus merely looked uncomfortable and Peter giggled nervously while I swiftly told James where to stick his idea. He was laughing so naturally I knew he wasn't being serious, but as the conversation moved onto other things I found myself, unwillingly, lingering on the subject.

Severus Snape's twin brother, Tristan, was not someone I had ever really considered before, except in passing. Of course as a Slytherin in our year he attracted the usual Marauder pranks and general scorn but most of our attention was naturally focused on our arch-nemesis- Snivellus.

Lucky for Tristan I suppose.

That night I lay awake for sometime, thinking over what I knew of him. Quiet and studious, Tristan Snape did not seem to be someone who rose to bait easily, generally ignoring our humorous jest and japes; he didn't play Quidditch so there was no rivalry there, and I had certainly never seen him as competition in the girl stakes. Mind you, there really wasn't anyone I considered competition there. I was almost 17- young and cocky, and pretty confident that I could get anyone I wanted: girl or boy. It made little difference to me at this point as I was hardly ever thinking further than a bit of necking and fondling in a broom cupboard… of course there had been one or two exceptions… Anyway nothing would even go that far in James's prank- I just had to get the guy to like me, and Merlin knew, I was good at that.

It was obvious the next morning that my friends had forgotten all about last night's joking, but as they chatted over breakfast I found my eyes scanning the Slytherin table searching for my quarry.

As luck would have it, he was just getting up to leave as I found him, delayed in chatting to some friend who was fumbling with his satchel. This afforded me a clear view across the rows of heads that separated the Gryffindor and Slytherin tables, and I took full advantage.

It was obvious at first glance that the Snape twins were not identical, they certainly didn't look that much like each other. This was a good thing as I don't think I could have coped with trying to seduce a carbon-copy of Snivellus- it would have been too wrong for words.

It seemed that Tristan had come out on top in the looks department too; also lucky because there was no way Snivellus was going to win any beauty awards in my opinion, not with his obnoxiously large nose and greasy hair. His brother, on the other hand, was not bad looking in a quiet kind of way- not a stand out beauty of course, but I had, in the past, had a thing for the quiet ones… Dark haired and dark eyed, in that he resembled Snivellus, but although his skin was pale it lacked the sallowness. And he had good teeth. This I had noticed before because I had a bit of a thing about teeth: there was no way I was sticking my tongue into an ugly mouth and that was that. No problem here though because Tristan's teeth were white and straight and perfect- excellent.

I'm not quite sure why I made up my mind to do this thing. My innate sense of mischief, perhaps, because frankly I could think of _nothing_ that would drive Snivellus up the wall more. (Well, except James getting with Lily and let's face it, that ain't gonna happen.) Or maybe I was just curious, eager to test my own prowess. Could it be _done_? Only one way to find out…

Well whatever the reason, I pondered about it all day and by the time it was just me and Prongs, Moony and Wormtail chilling by the fire in the common room, I was fully resolved to try it out.

"I'm gonna do it," I announced suddenly during a game of chess with James.

"Do what, mate?" he asked idly, scrutinising his pieces intently. He had no chance anyway, I was two moves off checkmate and he hadn't a clue. Chess was never really James's forte.

"Seduce Snape's brother of course, whaddya think?" I answered triumphantly, confident in the shocked responses I would get, and secretly amused. Peter squeaked in what was probably horrified excitement and James's head whipped up, jaw dropping. Behind me I heard Remus close his book with a thud. He said nothing though.

"Are you _serious_?" James asked. He looked gobsmacked, it was all too amusing. I laughed.

"Yeah sure, why not? Don't look so shocked mate, it was your idea." I grinned at his expression.

"Well yeah, Padfoot, but it was a _joke_. I wasn't actually serious!"

"Oh I know, but I am. Just think of Snivellus's face when he finds out. It will be incredible!"

"Playing around with people's feelings is never a joke, Sirius." Said someone quietly. It was Remus of course.

"Ah lighten up, Moony, this is _Snape _we're talking about." I said, swivelling to face him. "He hardly classifies as_ people_."

"And his brother?" Remus challenged coldly, his hands quite still on the front cover of his book. "Does he classify as a person, or is he just another unwitting pawn in your chess game of life?"

I smiled easily, not in the least bit bothered by Remus's attitude. He was always trying to act as my moral compass. "He's a Snape, same difference." I replied. My friend's expression altered not one jot so I decided to lighten the mood. "Pssh chill, Remus. I know you're just jealous really; wish I was seducing you instead, eh?" I gave him my best salacious wink and he rolled his eyes, reopening his book. "Oh, shut up, Sirius."

Conversation closed, it seemed. And argument won. Remus had never been able to argue with me.

James shook his head as I turned back to him. "You're a nutter, mate, but I'm not gonna argue with you."

"Good!" I said, leaning forward. "Wanna make things even more interesting?"

"A bet?" he asked, suspiciously.

"Ah ha, good to see we're on the same wavelength, Prongs me old chum. A bet indeed. Five galleons? If I can seduce him before, oh…, say, Easter?"

"Easter?" James gave me an incredulous look, "That's like six months! I hope you can act a bit quicker than that, Padfoot!"

"Oh fine!" I huffed. This would be tricky work, couldn't he see that? "How 'bout Valentine's Day? Nice and symbolic, don'tcha think?"

James rolled his eyes, echoing Remus's earlier gesture. "Alright, fine. Five galleons. But I have to tell ya, Sirius. Rather you than me on this one!"

But sitting there in the common room, in the warmth of the fire and surrounded by my friends, I felt brimful with confidence. The mission was on and I wasn't going to back down.

**So there's chapter one, hope you liked. I shall hopefully be back soon with an update, depending on how much stupid thesis gets done. See ya!**


	2. Chapter 2

**Disclaimer: Harry Potter and all its associated characters are © J. K. Rowling and Warner Bros. I'm not making any money from this little story. A shame really as I could do with some.**

**Author's note:** another day, another few ks on my thesis, another chapter here. Yay!

**Chapter Two: Potions and Plotting**

I'm the kind of guy who doesn't like to hang around, and so the next day I decided to start implementing my plan.

It was a Wednesday morning in mid-September, about a week and a half since the start of school, and Wednesday mornings meant NEWT Potions.

I was taking my NEWT in Potions mainly because I wanted to be an auror, but also because Slughorn was a pushover and the practical lessons meant there was plenty of time for scheming with my friends.

Plus it was another opportunity to torment Snivelly, and oh how I loved that.

Well anyway, one of the major components of 6th year Potions was a joint project with another student revolving the study of relatively obscure poisons and their antidotes. It was a semi-independent project, and heavy on the research. I had actually been quite looking forward to it. Today, Slughorn was finally assigning partners after several lessons looking at our different options and detailing what the project would entail,( mainly a lot of hard work, it seemed) and I was scheming about how to get the partner of my choice.

That is why I was unusually early for Potions that morning. It was my second class of the day and I had purposefully skipped morning break to head on down to the dungeons before anyone else.

Knocking, I waited for Slughorn's shout of "Enter!" before I pushed open the door and stepped inside.

"Ah! Sirius!" he said, evidently surprised to see me. I did not exactly make a habit of turning up early for lessons. "What can I do for you m'boy?"

"Well, professor," I answered with my usual easy smile, "I have come to you on a mission to promote inter-house relations."

Slughorn merely looked confused, so I continued. "Is it not true, sir that in today's lesson you are handing out project assignments?"

"Indeed, indeed it is…"

"And are you also assigning partners today as well?"

"Well yes, but I don't see-"

"It is al quite simple, sir," I said, interrupting smoothly. "I would very much appreciate it if you paired me with someone from Slytherin for the project."

Slughorn was momentarily dumb stuck and looked a trifle suspicious, so I ploughed on. "I was thinking Mr. Snape and I would get on well together."

Ah, now old Sluggy was definitely suspicious!

"You want me to pair you in a Potions project with_ Severus Snape_?! I really don't think that's a good idea, do you, m'boy?" he asked sternly.

"Merlin's beard, _no_, sir!" I said, looking horrified. "I want to promote school unity not start a war! I was thinking more along the lines of Tristan Snape."

"Tristan?!" Slughorn had evidently forgotten the existence of Severus' brother. Good to know it's not just me. "Hmmm… Tristan Snape…" the professor pondered. "… Well, yes. I suppose that may work. A less volatile sort of boy." He paused, giving me a hard look. "I have your word, Mr Black, that this is not just some stupid prank of yours? This project is a serious matter and will make up a substantial part of your year mark."

"Of course not, sir!" I affected my shocked tone. "I would never jeopardise my NEWT for a prank."

"Hmmm" Slughorn continued to hold me with a level stare, but evidently my earnest tone was enough to satisfy him as at length he grunted and nodded at me to go take my seat. "Very well, I will swap the assignments. But mark my words, Sirius, if this indeed some kind of a joke there will be repercussions."

Holding back my grin of triumph, I nodded and thanked him profusely. See, this is why I liked Slughorn- a pushover. A story like that, coming from me of all people, would never have won over McGonagall.

Well presently the rest of the class filtered in and Slughorn got to droning on about the project again, the importance of teamwork and individual initiative… yadda yadda… I mostly took this opportunity to stare around the room and doodle notes to James on my parchment. He was hoping to get partnered with Lily but I didn't like his chances.

Finally, Slughorn got on to announcing project assignments and partners, the moment we'd all been waiting for. Now was the time for me to sit up and listen.

"Firstly we have Miss Anam and Miss Fairbanks looking at the Draught of the Bleeding Heart." Two Ravenclaw girls grinned excitedly at each other.

"And Mr Berkenshaw and Miss Evans on the Djanology Tonic" I heard James curse under his breath and shoot Aidan Berkenshaw a murderous glance as the sandy haired Hufflepuff shifted seats to join Lily.

"Then looking at the properties of the Datura Toxic Broth we have Mr. Black and Mr. Snape" Slughorn droned. There was a shocked silence and the Snape twins swivelled as one to stare at me. Out of the corner of my eye I saw James shaking his head and grinning. It was Tristan who asked the burning question.

"Er, which one of us, Professor?"

Slughorn looked surprised at the question. "Well, you of course, Mr. Snape. Do you think I want a re-enactment of the Goblin Wars here in my classroom?"

There were a few titters from the class at Slughorn's amazing wit and Snivellus glared undying hatred at me. Tristan, however, looked merely resigned as he gathered his stuff together to move over next to me whilst Slughorn finished calling out the pairings. James got lucky and was paired with Moony and I neither noticed nor cared what poor sod got stuck with Snivellus.

"Hi!" I said brightly to Tristan as he sat down next to me. He turned and gave me an even look, his dark eyes very cool.

"Look Black, lets just get a few things straight. I'm no happier about this situation than you are- personally I think Slughorn must be out of his mind to partner me with you. Merlin knows, I must have offended him somehow. But- whatever. What's done is done. You don't have to like me; I really don't give a toss and I don't like you either. I am _not_ failing potions because of some prat like you, so don't piss about with me and mess this project up. Are we clear?"

I must admit I was somewhat taken aback. That little speech probably equalled the sum total of all the words Tristan had ever spoken to me in the past five years. I was always under the impression that not only was he quite shy, but he was little bit intimidated by me as well.

Evidently not.

"Oh, as a bell." I answered, still smiling. "No pissing about, got it."

He rolled his eyes, but then picked up the parchment Slughorn had passed round. "So what do you know of the Datura potion? I'm afraid I haven't read up on this, it wasn't the project I was expecting."

I shrugged. "An infusion of stinkweed and moonseed and some other nasty things I think. Takes six weeks to brew properly and that's in favourable conditions. It has some rather gruesome effects from what I've been reading: delirium, nausea, vomiting and convulsions amongst other delightful things. Can be fatal if the antidote is not administered quickly, though I think death is slow." No time for any small talk here, it seemed, Tristan was all business.

"How pleasant," he said softly, pouring over the assignment parchment. "Looks like a bitch to brew as well. Well I guess we'd better get sorting things out if we've got to have this and the antidote brewed and bottled before Christmas. If you go gather the ingredients, I think I'll start setting up here and get a study outline and timeline written out for us…"

"Why yes, sir!" I said with a mock salute, a small jibe to which he didn't rise but just handed me the list of ingredients. If this had been Severus then we would have come to blows now without a doubt. Remarkable.

Potions passed quickly and I have to admit I was rather impressed with Tristan's organisation and planning skills, the lack of overt animosity was refreshing too though he didn't try to talk to me about anything over than the project.

Leaving class he gave me a perfunctory nod and headed over to join his brother, who immediately grabbed his arm and started frantically questioning him. Tristan merely shrugged and walked out, with Severus shooting me a poisonous glare and quickly following.

"Blimey, Padfoot, you work fast!" James said as he and Moony joined me, slinging their bags over their shoulders. "I hope you know what you're doing!"

"Oh I very much do" I told them, humming to myself as we wandered out of the dungeons. Potions had gone much better than I'd hoped. Maybe this whole thing would be easier than I thought….

**Thanks for reading!**


	3. Chapter 3

**Disclaimer: Harry Potter and all its associated characters are © J. K. Rowling and Warner Bros. I'm not making any money from this little story. A shame really as I could do with some.**

**Chapter Three: Some problems encountered.**

The thing about seducing boys, I decided, was that I didn't really know where to start. I mean the only other time I had done anything with a lad I'd been more than slightly drunk on laughter, (washed down with generous amounts of firewhisky,) and I had felt distinctly awkward about it for weeks after. There had been no premeditated plan and certainly no seduction on my part.

Now, with a girl I was on much firmer ground. Be solicitous and ply them with a few compliments and it was amazing how quickly they fell into your arms. A box of chocolate hearts with some sappy poem on the label never went amiss either. Really, it was all very cliché.

With Tristan I felt like I was swimming in unknown waters- I didn't feel like I could randomly complement him on his gorgeous eyes, for example, or remark upon the delightful fragrance of his perfume. Somehow I thought such actions would scare him off me rather than like me. And, quite frankly, what with him being Slytherin, and so close to our sworn enemy, I didn't get much opportunity to practise.

Tristan was taking a fair few of the same N.E.W.Ts as me but we only shared classes in Defence Against the Dark Arts, Potions and Ancient Runes, since Transfiguration and Charms were such popular N.E.W.Ts that they remained one House only lessons. And out of these three, Potions was the only time I got to spend in any close proximity to him for in Defence Against the Dark Arts he was completely inaccessible in his Slytherin enclave and in quiet, theoretical Runes he sat nowhere near me.

So the end of September was drawing near and I was feeling more than a little frustrated. During Potions, the only damn time I got near him, Tristan stuck resolutely to the subject matter at hand and I could not engage him in any other topic. And believe me I tried! I wondered if he was always this focused on work or if it was just with me….

"So tell me about yourself," I said one lesson, whilst I was carefully stirring the cauldron and he was weighing out slug's entrails. "I feel like I know nothing about you and we've been schoolmates for six years."

"Hardly mates," he responded dryly, "And there's nothing I want you to know about me."

Ouch. Nonetheless I decided to keep trying.

"Well I really like playing chess" I continued nonchalantly, "though I'm bloody hard pressed to find a decent player in Gryffindor! Do you play?"

"Can you pass me the moth's wings" he said, without looking up. I sighed quietly in exasperation but complied.

"Not very chatty, are you?" I said, handing the box over.

"I think we have more important things to do than talk about chess."

"Ah, I'm sure we can chat and work at the same time. I do it with my friends all the time!"

Tristan exhaled loudly, and glanced over to me. "Look, Black-"

"Call me Sirius" I prompted.

"Why?" suspicion laced his tone

"Well… we have to work together all term… first name terms seems kinda appropriate."

He stared at me, eyes narrowed slightly. I found myself noticing how very dark his irises were, much deeper than Remus's golden brown, or James's hazel. But it was a warm darkness, full of hidden depths; so unlike the cold sinister eyes of his brother. I rather liked them.

"Fine," he snapped. "_Sirius_. When have I ever given any indication that I _wish_ to chat to you? It may have escaped your notice but I am _not_ your friend. I am only sitting _this_ _close_ to you under duress.

"If I'm being honest then your very presence pisses me off; you're a cock-sure, pig-headed bastard. After all the shit you've pulled in the last six years- _after all the times you and your cretinous friends have bullied and humiliated my brother!_- do you really think I have any desire to _chat_ with you? Or 'be your friend'?" he mimed speech marks with his fingers. I opened my mouth to interrupt, feeling that I really must protest, but ploughed on, hissing angrily. "The only reason-_the only reason_- I can tolerate working with you now is because my Potions N.E.W.T is so important to me; and the blessed knowledge that as soon as this project is done I will never have to speak to you or your cronies again. I can hardly fucking wait."

Seemingly done with his min-tirade, Tristan grabbed the moth's wings and began carefully counting them out. "I hope I've made myself clear" he muttered.

"Er… right..." I said, a little stunned. Then I glanced down at my bubbling cauldron. "Shit."

I was supposed to have been carefully stirring the mixture four times clockwise every two minutes, with an added turn counter-clockwise every six. Clearly I'd forgotten some counter-clockwise stirring because the liquid, which was supposed to be a clear green-blue at this point, had turned into a viscous murky yellow sludge.

"What?" Tristan snapped, and then got a look at our potion. "Oh, great. Just… fucking_… great_" he slammed down the moth wings in obvious frustration and stalked off across the classroom to talk to Severus.

I stared at the ruined potion for a few seconds before vanishing it in disgust. It seemed that everything was going wrong today.

"So I hear you pissed off Tristan _and_ ruined your potion today. _Bad luck_, mate." The huge grin on James's face slandered his words. I resisted the urge to throw a forkful of shepherd's pie at his head and shovelled it into my mouth instead.

"How'd you piss him off?" Peter asked.

"I asked him if he liked playing chess" I said, swallowing my food.

"Why, how perfectly obnoxious of you, Padfoot! I would have told you where to go as well!" I shot James a sour look.

"Why? What did he say exactly?" Peter leaned forward, eager for information. He had gratefully dropped potions at the end of our fifth year and now spent a blissful free period whilst the rest of us slaved away in the dungeons. I gave him a brief overview of events and he nodded thoughtfully.

"Well you can't really blame him, Padfoot" he said reasonably. "I mean, it's pretty much the truth."

"Yeah… well… yeah." I couldn't refute it and I didn't regret it because Snivellus so obvious had it all coming to him, but damn it was annoying! "But what can I do about it?"

"What you need to, Sirius," said Peter, leaning forward conspiratorially and lowering his voice, "is find some common ground."

"What you need to do is stop being ridiculous and leave the poor guy alone." Remus intervened. I ignored him.

"Common ground? With a Slytherin! That's a laugh!" James hooted, clearly enjoying himself. He was right though, Slytherin and members of its house represented everything that I despised with their pure blood mania and reminders of the family I'd left behind not three months before. What common ground could we have?

**Ooh… well next chapter Sirius does find some unexpected common ground with Tristan, but will the Slytherin warm to him?? Who knows…**


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